Neuramidase Inhibitors

Agents: oseltamivir, zanamivir

The neuramidase inhibitors are anti-influenza virus drugs that have activity against influenza A and B strains, unlike amandatine and rimantidine,  older drugs that are active only against influenza A strains. They work by preventing the viral neuramidase enzyme from releasing new virions from the host cell, preventing further replication. The two drugs differ in their form of delivery—oseltamivir is an oral pro-drug, while zanamivir is inhaled. They can be used in either the treatment of influenza or as prophylaxis for patients who cannot take the influenza vaccine.

Spectrum

Good: Influenza A and B

Poor: other viruses

Adverse Effects

Both oseltamivir and zanamivir are well-tolerated drugs. Oseltamivir can cause nausea, vomiting,  and abdominal pain, but these tend to be transient effects. Headache and fatigue can also occur, particularly during prophylactic use when the drug is given for a longer period of time. Zanamivir has mostly pulmonary adverse effects, including cough and bronchospasm. Avoid using it in patients with asthma or other reactive pulmonary diseases.

Important Facts

• Resistance to the neuramidase inhibitors can occur. Their utility is dictated by the degree of resistance that exists in the dominant influenza strains of the season. Currently, zanamivir is active against the vast majority of oseltamivir-resistant strains, but these resistance patterns may change.

• Neuramidase inhibitors are most effective when started early in the course of infection,  because viral replication peaks early (48–72 hours after infection). The package inserts for these drugs state they should be started in patients who have been symptomatic for no more than 2 days and should be given for 5 days, but clinicians do not always follow these guide-lines. In severe influenza infections such as those that require hospitalization, it may be worth extending the duration of treatment.

• Both of these drugs are highly effective at pre-venting influenza infection when the predominant strains in the community are susceptible,  but they are not substitutes for a vaccination strategy. Adverse effects are more common with the prolonged use seen with prophylactic use than with the shorter durations of therapeutic use.

What They’re Good For

Oseltamivir and zanamivir are both effective at treating and preventing influenza infections if the circulating strains are susceptible. The desired route of administration dictates the choice of agent.

Don’t Forget!

If your patient’s flu has peaked and he or she is im-proving, then it’s probably not the time to utilize one of these drugs. It may, however, be a good time to counsel on the utility of the influenza vaccine for next season.

References

Gallagher ,J.C. and MacDougall ,c. (2012). Antibiotics Simplified. Second Edition. Jones & Bartlett Learning, LLC.